Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electric signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
FIGS. 1A to 1E are cross sectional views illustrating a method for manufacturing a thin film actuated mirror array 100.
The process for the manufacture of the array 100 begins with the preparation of an active matrix 110 including a substrate 111, an array of switching devices 112, an array of connecting terminals 113, a passivation layer 114 and an etchant stopping layer 115, as shown in FIG. 1A. Each of the switching devices 112 made of, e.g., metal-oxide-semiconductor field effect transistors (MOSFET), is fabricated on top of the substrate 111 made of, e.g., Si-wafer, by using a common integrated-circuit process. Each of connecting terminals 113 made of a metal, e.g., tungsten (W), is electrically connected to a corresponding switching device in the switching device array 112. The passivation layer 114 made of, e.g., PSG, and having a thickness of 0.1-2 .mu.m, is formed on top of the switching device array 112 and the connecting terminal array 113 by using, e.g., a CVD or a spin coating method. The etchant stopping layer 115 made of silicon nitride and having a thickness of 0.1-2 .mu.m, is deposited on top of the passivation layer 114 by using a low pressure chemical vapor deposition (LPCVD) method or a plasma enhanced chemical vapor deposition (PECVD) method.
In a subsequently step, a thin film sacrificial layer 120, made of a phosphorus silicate glass (PSG) and having a flat top surface, is formed on top of the active matrix 110 by using a spin coating method. Since the thin film sacrificial layer 120 should endure the high-temperature process in the manufacturing process followed herebelow, the phosphor-silicate glass is chosen as a material for the thin film sacrificial layer 120. Then, an array of empty cavities 125 is created in the thin film sacrificial layer 120 to expose the active matrix 110 by using a dry or a wet etching method, as shown in FIG. 1B.
In a next step, an elastic layer 131, made of a nitride, e.g., silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the thin film sacrificial layer 120 including the empty cavities 125 by using a LPCVD or a PECVD method.
Thereafter, a lower thin film layer 132, made of an electrically conducting material, e.g., Pt or Pt/Ta, and having a thickness of 0.1-2 .mu.m, is formed on top of the elastic layer 131 by using a sputtering or a vacuum evaporation method.
Then, a thin film electrodisplacive layer 133, made of a piezoelectric material, e.g., PZT, or an electrostrictive material, e.g., PMN, and having a thickness of 0.1-2 .mu.m, is deposited on top of the lower thin film layer 132 by using an evaporation, a sol-gel, a sputtering or a CVD method.
Subsequently, an upper thin film layer 134, made of an electrically conducting and light reflecting material, e.g., aluminum (Al) or silver (Ag), and having a thickness of 0.1-2 .mu.m, is formed on top of the thin film electrodisplacive layer 133 by using a sputtering method, thereby forming a multiple layered structure 130, as shown in FIG. 1C.
In an ensuing step, the multiple layered structure 130 is patterned by using a photolithography or a laser trimming method to expose the thin film sacrificial layer 120.
In a subsequent step, an array of via contacts 145 made of a metal, e.g., tungsten(W), is formed by using a lift-off method, thereby forming an array of actuating structures 140, wherein each of actuating structures 140 includes an upper thin film electrode 144, a thin film electrodisplacive member 143, a lower thin film electrode 142, an elastic member 141 and a via contact 145, as shown in FIG. 1D. Each of the via contacts 145 extends from top of the lower thin film electrode 142 to top of a corresponding connecting terminal in the connecting terminal array 113. Each of the lower thin film electrodes 142 is electrically connected to the corresponding connecting terminal 113, thereby functioning as a signal electrode and each of the upper thin film electrodes 144 is electrically connected to ground, thereby functioning as a common bias electrode as well as a mirror.
Finally, the thin film sacrificial layer 120 is removed by using a wet etching method using an etchant or a chemical, e.g., hydrogen fluoride (HF) vapor, to thereby form the thin film actuated mirror array 100, as shown in FIG. 1E.
One of the problems in using the above-described the method for manufacturing the thin film actuated mirror array is the overall optical efficiency of the resulting thin film actuated mirrors. When each of the thin film actuated mirrors deforms in response to an electric field applied across the thin film electrodisplacive member therein, the upper thin film electrode attached thereto, which also acts as a mirror, also deforms to thereby, instead of creating a planar top surface, create a curved top surface from which the light beams are reflected, reducing the effectiveness of the upper thin film electrode in reflecting the light beams. As a result, the overall optical efficiency of the thin film actuated mirror array decreases.
To overcome the above problem of the method for manufacturing the thin film actuated mirror array, it has been suggested to incorporate in the thin film actuated mirror a tilting mirror that is formed and cantilevered on top of the actuating structures by using a second thin film sacrificial layer.